1. Field of the Invention
THis invention relates generally to video projectors, and more specifically to liquid crystal based video projection systems.
2. Description of the Prior Art
Large screen televisions presently employ three basic, alternative technologies. The first is large screen direct view cathode ray tubes (CRTs) reaching 35" (diagonal), the second is rear projection, and thr third is front projection on to a screen, much like the familiar movie projector. Direct view CRT screens larger than 35" are extremely expensive, and do not sell well into the consumer market as a result of the expense. Both rear and front projection television and video systems traditionally use CRTs. However, since the light produced by a CRT is coming from phosphorescence, the final light level is very limited by having to spread the light over an area up to ten times larger that the area of the CRT. One solution that has become very popular is to use three separate CRTs, one red, one green, and one blue. Monochrome CRTs can be forced to emit much more light than a single color CRT, because the electron shadow mask in back of the screen phosphors can be eliminated and far more electrons will strike the phosphors, which in turn produces more light. The three colors are then combined with lenses to form color images. And since the color image is the product of three very bright CRTs, the combination is as much as ten times brighter than was possible before. Even so, CRTs have limits, and new ways have been found to further increase projection light levels.
Liquid crystal panels, similar to LCD watches, emit no light on their own, but will block light shining through. In an LCD watch, a small light bolt is placed behind the LCD panel, and a switch will turn it on for viewing in the dark. In daylight, the LCD will reflect sunlight or indoor light and is very readable. Video projection systems using liquid crystal panels have begun to appear in commercial products sold in the United States, e.g. by Sharp Corporation (Japan). These systems typically place a high output metal halide lamp behind a liquid crystal panel with a video image and project that image up onto a screen using a system of lenses. An Oct. 1989 article by S. Kohzai, et al., describes a liquid crystal video projection system having a metal halide projection lamp, dichroic mirrors, three liquid crystal panels and associated lenses to produce full-color large-screen video. (International Televison Engineering Journal (ITEJ) Technical Report Vol. 13, No. 53, pp.49-54.)
Prior art video projection systems are typically constructed as is shown in FIG. 1. A video projection system referred to by the general reference numeral 10, is comprised of an on/off switch 12 connected to a power supply 14, a cooling fan 16, a projection lamp 18 with an over-temperature bimetal thermostat 20, a control module 22, an input module 24, a liquid crystal light valve 26, a lens unit 28, and a projection screen 29. Light from projection lamp 18 shines through an image formed on liquid crystal light valve 26 causing a projected image to be focussed by lens unit 28 on to screeen 29. The fan 16 forces cooling air through system 10, but whenever the airflow is blocked, system 10 will overheat as a result of the large amount of heat being dissipated internally by projection lamp 18. The over-temperature thermostat 20 is designed to trip at abnormally high heat and thus shut off the projection lamp 18. This action prevents damage to system 10 by oveheating. Airflow through system 10 can be inadvertently blocked and no warning that the thermostat 20 is about to trip is given. No outside indication is given that thermostat 20 has tripped off. A user could wrongly asume that the projection lamp 18 has burnt out and needs replacing.
The volume, picture, signal input, and lens settings of prior art projection systems usually require manual adjustment at the control module 22, via input module 24. This leads to inconvenience, because the positions of the screen, the video projector, and the user are normally several feet apart. Users must therfore move over to the projector system in order to adjust it. This will usually prohibit placing such projector systems out of reach, e.g., on the ceiling of a theater, bar, or restaurant.